Hitherto, an image forming method based on a thermal transfer process has been known as one of the color or monochromic image forming techniques. The method has widely been used as a means making it possible to give high quality images with ease. The thermal transfer process is a process of preparing a thermal transfer sheet having a dye exhibiting a specific thermophysical property and using a thermal printing means such as a thermal head or a laser to transfer the dye from the thermal transfer sheet to a thermal transfer image receiving sheet, thereby forming an image. Such a thermal transfer process has advantages that the size of devices can be reduced and costs can be also reduced.
The thermal transfer process is roughly classified into two manners of a thermofusion manner and a thermal diffusion transfer manner on the basis for the mechanism of transferring a dye from a thermal transfer sheet to a thermal transfer image receiving sheet. The thermofusion manner is a manner of using a thermal transfer sheet having a thermally-melting dye and transferring the thermally-melting dye onto a thermal transfer image receiving sheet using the melting transfer mechanism occurred by thermal treatment, thereby forming an image. On the other hand, the thermal diffusion transfer manner is a manner of using a thermal transfer sheet having a thermally-diffusing dye and transferring the thermally-diffusing dye onto a thermal transfer image receiving sheet using the thermal diffusion transfer mechanism occurred by thermal treatment, thereby forming an image.
In the thermal diffusion transfer manner, the amount of the thermally-diffusing dye transferred onto the thermal transfer image receiving sheet can be arbitrarily adjusted by controlling the degree of the heating added to the thermal transfer sheet; therefore, the manner has features that an image which is excellent in reproducibility of intermediate colors and has a fine gradation can be formed and full color images are advantageously formed. Because of such advantages, thermal transfer technique in the thermal diffusion transfer manner is widely used in photographs for business use, printers for personal computers, video printers, and others.
Incidentally, a thermal transfer image receiving sheet used in such a thermal transfer manner is required to exhibit excellent releasability regarding a thermal transfer sheet in order to form a highly fine image. If its image receiving layer is low in releasability, a dye binder in the thermal transfer sheet is easily melted and bonded onto the image receiving layer. Thus, at the time of printing, release sounds may become large. As the case may be, the dye binder is completely melted and bonded thereto so as to cause a problem that a printed matter is not normally discharged from the printer, and other problems.
In particular, in the thermal transfer manner, images are formed ordinarily by subtractive color process; thus, dyes of yellow, magenta, and cyan are successively printed onto a thermal transfer image receiving sheet. Accordingly, onto the thermal transfer image receiving sheet, at least three printing processes are performed. It is therefore insufficient that the sheet is merely excellent in releasability, and it is necessary that the sheet has such release stability that excellent releasability can be kept in any one of the three printing processes. When a protective layer is transferred after the formation of the image in order to give durability thereto, it is necessary that the sheet is excellent in releasability in the three printing processes and further has adhesiveness onto the protective layer to be formed thereafter.
As a method of improving such releasability and release stability, there is generally used a method of incorporating a releasing agent having a function of improving the releasability into the image receiving layer.
Patent Document 1 discloses a method of adding a releasing agent made of a silicone oil to an image receiving layer in order to improve the releasability between a thermal transfer image receiving sheet and a thermal transfer sheet. Such a method makes it possible to improve the releasability because of the use of the silicone oil; however, the method has a problem that the release stability is insufficient. Moreover, when the image receiving layer is formed by a melt-extrusion process, the silicone oil bleeds out largely in the process so as to cause a problem of lowering the image quality of a printed image.
Furthermore, in the above-mentioned thermal transfer manner, a print image high in image quality is formed onto an image receiving layer at a high-speed. Usually, therefore, there is used a thermal transfer image receiving sheet, in which an image receiving layer made mainly of a resin which can be dyed (or a resin to which a dye can be bonded) is formed on a substrate sheet. In the case of using, as the substrate sheet, a piece of coated paper, art paper or the like, which has a relatively high thermal conductivity, there remains a problem that the sheet is low in sensitivity in receiving an image forming dye.
As a countermeasure against such problems, it is known as described in Patent Document 2 that the following film is used as the substrate of an image receiving layer: a bi-axially drawn film which is made mainly of a thermoplastic resin, such as polyolefin, and has voids or pores. The image receiving layer, in which such a film is used as its substrate has advantages that homogeneous and highly-densed images can be obtained since the sheet has an even thickness, flexibility and a smaller thermal conductivity than paper made of cellulose fiber, and others. However, the use of the film gives disadvantages that the formation of an image receiving layer, and the lamination thereof onto a core member, and other processes are further required so that the production efficiency is insufficient and product costs also increase largely.
Patent Document 3 describes a thermal transfer image receiving member which is used in combination with a dye-supplying material which contains a thermally-transferable dye and which has an image receiving layer for receiving the thermally-transferred dye, in which the image receiving layer is a layer of a film obtained by forming a polyester or resins made mainly of a polyester into a film by melt-extrusion and then drawing the film at a draw ratio by area of 1.2 to 3.6 (inclusive). However, the above-mentioned image receiving layer is insufficient in thermal insulation performance since the layer has no pores or voids. Thus, in the printed matter, to which an image is formed, the printed image density is not at a satisfactory level.    Patent Document 1: Japanese Patent Application Laid-Open No. 2001-030639    Patent Document 2: Japanese Patent Application Laid-Open No. 5-16539    Patent Document 3: Japanese Patent Application Laid-Open No. 9-1943